<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>大页面</title>
  <style>
    .box {
      width: 400px;
      height: 400px;
      font-size: 80px;
      line-height: 400px;
      text-align: center;
      color: gold;
      background-color: darkcyan;
    }
  </style>
</head>

<body>
  <div class="box">大页面</div>

  <script>
    // 自适应页面切换
    window.onresize = function () {

      if (window.innerWidth <= 800)

        location.href = './小页面.html'
    }
  </script>
</body>

</html>